The molecular complex associated with the Galactic H II region Sh2-90: a possible site of triggered star formation

Abstract

Aims: We investigate the star formation activity in the molecular complex associated with the Galactic H ii region Sh2-90.
Methods: We obtain the distribution of the ionized and cold neutral gas using radio-continuum and Herschel observations. We use near-infrared and Spitzer data to investigate the stellar content of the complex. We discuss the evolutionary status of embedded massive young stellar objects (MYSOs) using their spectral energy distribution.
Results: The Sh2-90 region presents a bubble morphology in the mid-infrared. Radio observations suggest it is an evolved H ii region with an electron density ~144 cm^-3, emission measure ~ 6.7 × 10⁴ cm^-6 pc and an ionized mass ~55 M_⊙. From Herschel and CO (J = 3 - 2) observations we found that the H ii region is part of an elongated extended molecular cloud (H₂ column density ≥ 3 × 10²¹ cm^-2 and dust temperature 18-27 K) of total mass ≥ 1 × 10⁴ M_⊙. We identify the ionizing cluster of Sh2-90, the main exciting star being an O8-O9 V star. Five cold dust clumps, four mid-IR blobs around B stars, and a compact H ii region are found at the edge of the bubble. The velocity information derived from CO data cubes suggest that most of them are associated with the Sh2-90 region. One hundred and twenty-nine low mass (≤3 M_⊙) YSOs have been identified, and they are found to be distributed mostly in the regions of high column density. Four candidate Class 0/I MYSOs have been found. We suggest that multi-generation star formation is present in the complex. From evidence of interaction, time scales involved, and evolutionary status of stellar/protostellar sources, we argue that the star formation at the edges of Sh2-90 might have been triggered. However, several young sources in this complex are probably formed by some other processes.

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